(1) Field of the Invention
The present invention relates to an antireflective coating which can easily be formed and which is excellent in durability, heat resistance and impact resistance and has a good dyestuff permeability, and also to a transparent material having an excellent antireflective property, which has this antireflective coating on the surface thereof.
(2) Description of the Prior Art
When a thing is viewed through a transparent material, if reflected light is intense and a reflected image is clear, the observation is annoying. For example, in the case of an eyeglass lens, a reflected image called "ghost" or "flare" is formed and gives an unpleasant feeling to the eyes. Moreover, in case of a looking glass or the like, the object is blurred by reflected light on the glass surface.
As means for preventing reflection, there has heretofore been adopted a method in which a coating of a material having a refractive index different from that of the substrate is formed on the substrate by vacuum evaporation deposition or the like. It is known that in order to enhance the reflection-preventive effect in this method, selection of the film thickness of the material covering the substrate is very important. For example, it is known that in case of a monolayer coating film, if the optical thickness of the substance having a lower refractive index than that of the substrate is adjusted to .theta. of the wavelength of the objective ray or an odd number of times thereof, a minimum reflectance, that is, a maximum transmittance, is obtained. By the term "optical thickness" used herein is meant the product of the refractive index of the film-forming coating material and the thickness of the coating film.
Furthermore, formation of a multilayer antireflective coating is also known, and several proposals have been made in connection with selection of the thickness [see "Optics of Thin Films", pages 159-283, A. Vasicek (North-Holland Publishing Company), Amsterdam (1960)].
These antireflective coating films formed by vacuum evaporation deposition involve the following problems in some application fields.
(1) Since a high degree of vacuum is necessary, the size and material of the substrate are restricted. Moreover, the manufacturing time is prolonged, resulting in reduction of the productivity and increase of the manufacturing cost.
(2) Heating to a considerable extent is ordinarily necessary, and in some substrates, deformation or degradation is caused.
(3) The film-forming coating material used is in principle an inorganic oxide. Although an inorganic oxide gives a compact and dense film, if a plastic material is used as the substrate, reduction of the heat resistance and adhesion is caused owing to the difference in the coefficient of linear expansion between the coating film and the substrate.
(4) A dyestuff permeability, which is necessary for dyeing as effective means for coloring the transparent substrate, is completely lost.
(5) Also in a glass covered with a dyeable material, loss of the dyeability and reduction of the heat resistance and adhesion are similarly caused.
As means for rendering the surface of an optical element substantially non-reflecting without adopting the vacuum evaporation deposition technique, there can be mentioned a method in which a coating having fine particles is formed (see U.S. Pat. No. 2,536,764) and a method in which an optical element of polymeric material is provided with a microstructured surface (see U.S. Pat. No. 4,114,983). In these methods, reflected light is scattered to render the reflected image dim. However, the light transmitted through the transparent material is also scattered, and therefore, the effect of improving the transmittance while reducing the reflectance, as attained in the above-mentioned coating film obtained by vacuum evaporation deposition, cannot be attained. Furthermore, there is known a method in which a silicon coating is formed on a plastic substrate and then subjected to plasma polymerization to attain an antireflective effect (see U.S. Pat. No. 4,137,365). However, the dyestuff permeability is lost and since the coating film is formed from the gas phase, as in the vacuum evaporation deposition method, the productivity is low and the production cost is high.
We previously proposed an antireflective coating film having a dyestuff permeability, which is formed by treating an organic film containing inorganic fine particles with an activating gas (see U.S. Pat. No. 4,374,158). This coating film, however, is insufficient in heat resistance and water resistance at high temperatures.
Moreover, there is known an antireflective coating film for a solar cell, formed by the liquid two-layer coating method in which TiO.sub.2 --SiO.sub.2 --forming compounds are used for the first layer and SiO.sub.2 --forming compounds are used for the second layer (see Applied Optics, Vol. 18, No. 18, pages 3133-3138). This antireflective coating film, like the coating film formed by the vacuum evaporation deposition method, has no dyestuff permeability, and this antireflective coating film is readily cracked or broken by thermal or mechanical deformation.